1. Field of the Invention
This invention relates to the conversion of chemical energy to electrical energy. More particularly, this invention relates to the preparation of an improved cathode active material for non-aqueous lithium electrochemical cells, and still more particularly, a cathode active xcex5-phase silver vanadium oxide (SVO, Ag2V4O11) prepared using a xcex3-phase silver vanadium oxide (Ag1.2V3O8.1) starting material. The product cathode active material can be used in an implantable electrochemical cell, for example of the type powering a cardiac defibrillator, where the cell may run under a light load for significant periods interrupted from time to time by high rate pulse discharges.
The reaction of xcex3-phase SVO with a source of silver produces xcex5-phase SVO that possesses a lower surface area than SVO produced from other vanadium-containing starting materials. The relatively low surface area of this new xcex5-phase SVO material results in greater long term stability for the cathode active material in comparison to other forms of SVO with higher specific surfaces areas.
2. Prior Art
The prior art discloses many processes for manufacturing SVO; however, they result in a product with greater surface area than the material prepared by the current invention.
Specifically, U.S. Pat. No. 4,391,729 to Liang et al. discloses the preparation of silver vanadium oxide by a thermal decomposition reaction of silver nitrate with vanadium oxide conducted under an air atmosphere. This decomposition reaction is further detailed in the publication: Leising, R. A.; Takeuchi, E. S. Chem. Mater. 1993, 5, 738-742, where the synthesis of SVO from silver nitrate and vanadium oxide under an air atmosphere is presented as a function of temperature. In another reference: Leising, R. A.; Takeuchi, E. S. Chem. Mater. 1994, 6, 489-495, the synthesis of SVO from different silver precursor materials (silver nitrate, silver nitrite, silver oxide, silver vanadate, and silver carbonate) is described. The product active materials of this latter publication are consistent with the formation of a mixture of SVO phases prepared under argon, which is not solely xcex5-phase Ag2V4O11.
Also, the preparation of SVO from silver oxide and vanadium oxide is well documented in the literature. In the publications: Fleury, P.; Kohlmuller, R. C. R. Acad. Sci. Paris 1966, 262C, 475-477, and Casalot, A.; Pouchard, M. Bull Soc. Chim. Fr. 1967, 3817-3820, the reaction of silver oxide with vanadium oxide is described. Wenda, E. J. Thermal Anal. 1985, 30, 89-887, present the phase diagram of the V2O5-Ag2O system in which the starting materials are heated under oxygen to form SVO, among other materials. Thus, Fleury and Kohlmuller teach that the heat treatment of starting materials under a non-oxidizing atmosphere (such as argon) results in the formation of SVO with a reduced silver content.
In U.S. Pat. No. 5,955,218 to Crespi et al., the process of heat-treating SVO prepared by a thermal decomposition reaction to improve the electrochemical performance of the material is disclosed. In this patent, thermal decomposition SVO prepared according to U.S. Pat. Nos. 4,310,609 and 4,391,729, both to Liang et al., under an air atmosphere at a somewhat lower temperature of 360xc2x0 C. is described. However, the ""218 patent to Crespi et al. demonstrates that adding a second heat treatment step increases the crystallinity of the resulting active material. The present invention is concerned with the product active material""s surface area, and not necessarily its crystallinity.
U.S. Pat. No. 5,221,453 to Crespi teaches a method for making an electrochemical cell containing SVO, in which the cathode active material is prepared by a chemical addition reaction of an admixed 2:1 mole ratio of AgVO3 and V2O5 heated in the range of 300xc2x0 C. to 700xc2x0 C. for a period of 5 to 24 hours. Crespi does not discuss xcex3-phase SVO in the context of this invention. Therefore, the xcex5-phase material described by the current invention could not be manufactured by this process.
U.S. Pat. Nos. 6,130,005 and 5,955,218, both to Crespi et al., relate to heat treated silver vanadium oxide materials, for example, xcex3-phase SVO heat treated to form decomposition-produced SVO (dSVO). The starting material does not appear to be heated for further combination with a source of silver or other metal. Also, U.S. Pat. No. 5,895,733 to Crespi et al. shows a method for synthesizing SVO by using AgO and a vanadium oxide as starting materials. However, the result is not a low surface area xcex5-phase SVO cathode material, as disclosed in the current invention.
U.S. Pat. No. 5,545,497 to Takeuchi et al. teaches cathode materials having the general formula of AgxV2Oy. Suitable materials comprise a xcex2-phase SVO having in the general formula x=0.35 and y=5.18 and a xcex3-phase SVO having x=0.74 and y=5.37, or a mixture of the phases thereof. Such SVO materials are produced by the thermal decomposition of a silver salt in the presence of vanadium pentoxide. In addition, U.S. Pat. No. 6,171,729 to Gan et al. shows exemplary alkali metal/solid cathode electrochemical cells in which the cathode may be an SVO of xcex2-, xcex3- or xcex5-phase materials. However, none of Gan et al.""s methods are capable of producing a low surface area xcex5-phase cathode material, as per the current invention.
Therefore, based on the prior art, there is a need to develop a process for the synthesis of mixed metal oxides, including silver vanadium oxide, having a relatively low surface area. An example is a low surface area SVO prepared using a silver-containing compound and xcex3-phase SVO as starting materials. The product xcex5-phase SVO is a cathode active material useful for non-aqueous electrochemical cells having enhanced characteristics, including the high pulse capability necessary for use with cardiac defibrillators.
The current invention relates to the preparation of an improved cathode active material for non-aqueous lithium electrochemical cells, and in particular, a cathode active material that contains xcex5-phase SVO prepared using a xcex3-phase SVO starting material. The reaction of xcex3-phase SVO with a source of silver produces xcex5-phase SVO possessing a lower surface area than xcex5-phase SVO produced from other vanadium-containing starting materials. The present synthesis technique is not, however, limited to silver salts since salts of copper, magnesium and manganese can be used to produce relatively low surface are metal oxide active materials as well. The relatively low surface area of the xcex5-phase SVO material provides an advantage in greater long term stability when used as an active cathode material compared to SVO with a higher specific surface area.
The current invention discloses that reacting a xcex3-phase SVO material with a source of silver, or other suitable metal salt, produces pure xcex5-phase SVO (Ag2V4O11). This product material possesses a relatively lower surface areas in comparison to active materials synthesized by a thermal decomposition reaction under an oxidizing atmosphere. Decreased surface area is an unexpected result.
The thermal reaction of silver nitrate with vanadium oxide under an air atmosphere is a typical example of the preparation of silver vanadium oxide by a decomposition reaction. This reaction is set forth below in Equation 1:
2AgNO3+2V2O5xe2x86x92Ag2V4O11+2NOxxe2x80x83xe2x80x83(1)
The physical characteristics of SVO material (i.e. particle morphology, surface area, crystallinity, etc.) produced by this reaction are dependent on the temperature and time of reaction. In addition, the reaction environment has a dramatic effect on the product material. The same reaction of silver nitrate with vanadium oxide conducted under an argon atmosphere is depicted below in Equation 2:
2AgNO3+2V2O5xe2x86x92AgVO3+Ag1.2V3O8+2NOxxe2x80x83xe2x80x83(2)
Thus, the synthesis of SVO under an inert atmosphere results in the formation of a mixture of silver vanadate (AgVO3) and xcex3-phase SVO (Ag1.2V3O8). This is described in the above-referenced publication by Leising, R. A.; Takeuchi, E. S. Chem. Mater. 1994, 6, 489-495. As reported by Leising et al., a mixture of material phases is less suitable than a single xcex5-phase SVO (Ag2V4O11) as a cathode active material for lithium electrochemical cells. For this reason, argon is typically not preferred for synthesis of SVO cathode active material.
A more benign preparation technique for xcex5-phase SVO from vanadium oxide and silver carbonate (Ag2CO3) according to Equation 3 below results in the release of CO2 gas, which is a nontoxic byproduct. However, the specific surface area of the product SVO is also higher than the surface area of SVO prepared from silver nitrate. This is shown below in Table 1.
Ag2CO3+2V2O5xe2x86x92Ag2V4O11+CO2xe2x80x83xe2x80x83(3)
Thus, a synthesis technique for SVO using vanadium oxide and either silver oxide or silver carbonate, or other preferred metal salts, while eliminating the formation of toxic NOx byproduct, results in an SVO material with a higher specific surface area than SVO produced from vanadium oxide and silver nitrate.